Fast Models Reference Manual : 5.3.4 Creating a new branch predictor

5.3.4 Creating a new branch predictor

The example predictors that Arm provides are intended to show different techniques for branch prediction to help you create new ones. The source code for them is located in $PVLIB_HOME/plugins/source/BranchPrediction/.

A user-defined branch predictor must:

Derive from the abstract interface class PredictorInterface.
Implement the following functions:
- ```
Direction getPredictedDirection(VAddr64_t pc_) SG_OVERRIDE;
```
  Returns the predicted branch direction, that is, taken or not taken.
- ```
void updateStructures(VAddr64_t pc_, Direction actual_direction_) SG_OVERRIDE;
```
  Updates any tables or structures that the user-defined predictor implements, based on the actual branch direction.
Implement the following static functions:
- ```
static const char* getPredictorName()
```
  Returns the name of the predictor. This is displayed in the log file.
- ```
static PredictorInterface* createInstance()
```
  Creates an instance of the predictor.

void registerPredictors(){
      // All predictor types must be registered here      
      map_predictor_instances[SamplePredictor::getPredictorName()] =
          &SamplePredictor::createInstance;
      …
}

Example 5-1 Branch predictor

// SamplePredictor.h
#include <ct/BranchPredictionPluginInterfaces.h>
#include "PredictorInterface.h"

namespace BranchPrediction
{
    class SamplePredictor : public PredictorInterface
    {
    public:
        Direction getPredictedDirection(VAddr64_t pc_) SG_OVERRIDE;
        void updateStructures(VAddr64_t pc_, Direction actual_direction_) SG_OVERRIDE;

    public:
        SamplePredictor(Direction dir_ = Direction::TAKEN);
        ~ SamplePredictor();

        static const char* getPredictorName() {
            return "SamplePredictor";
        }

        static PredictorInterface* createInstance() {
            return new SamplePredictor();
        }

    private:
        Direction default_direction;
    };
};


// SamplePredictor.cpp
#include "SamplePredictor.h"

namespace BranchPrediction
{

    SamplePredictor::SamplePredictor()
    {
        // Initialize the predictor
    }

    SamplePredictor::~SamplePredictor()
    {
        // Do nothing
    }

    Direction SamplePredictor::getPredictedDirection(VAddr64_t pc_)
    {
        // Return the predicted direction
        // This is called when the CPU makes a branch prediction
        return Direction::TAKEN;
    }

    void SamplePredictor::updateStructures(VAddr64_t pc_, Direction actual_direction_)
    {
        // Update any structure or tables that this predictor uses
        // This is called when the branch has committed and the direction 
        // taken is known.
    }
};

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